You have a few options:
1. Get an AC/DC converter to put out enough current at 12v to power the car amp - pretty expensive
2. Get a sub amp from partsexpress - will have a crossover, some even have a remote so you can adjust the ouput
3. Use an existing channel/channels from your current receiver and build a passive crossover to supply only bass to the subs

Those are the only threee "correct" ways I could come up with - in 2 minutes - on how to do this. You could try some computer sub amplifier or a range of other things, but in the end, option 2 is probably the most cost effective solution. You can also re-use it down the road to build your own home theater sub if you want to - there are plenty of drivers out there for under 150 bucks that would make a killer ht sub.